Pump-down pressure plug

ABSTRACT

A pressure plug system comprising a pressure plug with a tapered section, a receiving landing receptacle having a tapered section with substantially the same tapering angle as tapered section of the pressure plug, and tapering angle being less than the friction angle corresponding to the coefficient of friction between the pressure plug and the landing receptacle. The pressure plug engages the receiving landing receptacle providing reliable seal and locking the plug against landing receptacle preventing axial movement under cycling pressure and rotation of the plug during drillout.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional of U.S. Application Ser. No. 60/818,875 filed on Jul. 6, 2006, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to tools for use in drilling of oil and gas wells and more particularly to pump-down pressure plugs for use in tubular expansion tools.

2. Background of the Invention

Pressure plugs, such as pump-down plugs and wiper plugs, are frequently used in operations in which there is a need for creating pressure chambers or separation of fluids. For example, in a conventional tubular expansion process, the expandable tubular string is deployed in the well on a drill string with the bottom of the remainder segment of the tubular string, usually referred as a bottom shoe, being open allowing pumping cement slurry into the wellbore annulus or circulation of drilling fluids. Commencement of the expansion process requires the bottom shoe to be pressure sealed to provide a pressure chamber between the shoe and the expansion swage. This is usually done by pumping a pressure plug through the drill pipe which lands in the landing receptacle of the shoe. The pressure liquid is pumped in the chamber between the shoe and the expansion swage which urges the swage to propagate through the tubular and expand the tubular. During this process, the drill pipe is attached to the swage and moves out of the well. At the end of every stand of drill pipe, the pressure is released, the stand is removed, and then pressure is reapplied. These cycles continue until the entire tubular string is expanded.

A typical conventional pressure plug has several elastic cups for wiping the drill pipe and/or for providing a pressure differential for the plug to be pumped down. The front portion of the plug includes a so called nose section having a forward end and back end. The nose section is tapered to provide a stop shoulder. A pair of annular grooves is provided for seal rings. The back end of the nose has a recess in which is disposed a latch ring. In operation, the pressure plug is pumped down through the drill pipe until it is latched in a landing receptacle in the shoe in which the forward motion of the plug is limited by the stop shoulder and the rearward displacement is limited by the latch ring. The conventional pressure plugs are generally reliable when used in conventional cementing operations. However, using conventional pressure plugs in tubular expansion operations has caused problems due to the plug's inherent slack movement between its lower position defined by the stop shoulder against the landing receptacle and its upper position defined by the latch ring and the receiving groove in landing receptacle. This slack displacement of the plug between its upper and lower positions is necessary to ensure the latching of the latch ring in the receiving groove. During the tubular expansion process, the cycles of pressurization and depressurization of the system cause movements of the plug between its upper and lower positions which may result in loss of the seal provided by the rings and jeopardize the expansion process. A failed expansion process often requires an expensive sidetracking of the well or abandonment.

After expansion of the tubular, the shoe with the pressure plug needs to be drilled-out to continue drilling the well. It is desirable to prevent the rotation of the plug during drillout in order to considerably reduce drillout time. The conventional anti-rotational plugs include locking clutches in the nosepiece and in the landing receptacle in the shoe. The locking clutches prevent the rotation of the plug in the shoe but occasionally interfere with the latching of the plugs and may allow fluid back-flow and impede the sealing of the plug.

Several solutions of these problems, such as using multiple plug systems, have been attempted, but, none have proven satisfactory. Consequently, there is a need for a pump-down pressure plug system which provides a reliable seal capable of withstanding multiple high pressure cycles. There is a further need for a non-rotational plug that provides fast drillout.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS

These and other needs in the art are addressed in one embodiment by a pressure plug system comprising a pressure plug with a tapered section, a receiving landing receptacle having a tapered section with substantially the same tapering angle as tapered section of the pressure plug, and tapering angle being less than the friction angle corresponding to the coefficient of friction between the pressure plug and the landing receptacle. The pressure plug engages the receiving landing receptacle providing a reliable seal and locking the plug against landing receptacle preventing axial movement under cycling pressure and rotation of the plug during drillout.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

FIG. 1 shows a typical prior art pump-down pressure plug.

FIG. 2 shows a cross-sectional view of a pressure plug in accordance with the present invention.

FIG. 3 shows a cross-sectional view of a pressure plug engaged in a landing receptacle in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows main geometrical features of the pump-down pressure plug 20 according to the present invention. The pressure plug 20 comprises a nose section 10, having a forward end 12 and a rearward end 18. The plug 20 has a conical section of the length L which is tapered towards the front 12 under the tapering angle α. The plug 20 also comprises an optional cylindrical section 15 of a constant diameter D with a pair of external annular grooves in which are disposed a pair of seal rings such as O-rings 16. A pair of rubber cups 14 is disposed on the rod as shown and are abutted against the rearward side of the plug nose 18.

FIG. 3 shows the pressure plug system with the pressure plug 20 being engaged in the landing receptacle 30 having a tapered section 31 under the same angle α as the plug 20, and an optional cylindrical section 32 of a constant diameter. During normal operation the plug 20 is pumped down through the drill pipe until the plug is landed in the landing receptacle 30. Under the applied pressure P_(t), a certain interference stress between the plug 20 and the landing receptacle 30 in the tapered section 31 is developed. Then, if the tapering angle α is substantially small, the plug will be locked in the landing receptacle, since it will be so firmly seated in the landing receptacle that there will be considerable frictional resistance to any force tending to remove or rotate the plug relative to the landing receptacle.

In general, in order to achieve locking of the plug in the landing receptacle, the tapering angle, α, should be less than the friction angle corresponding to the coefficient of friction between the plug and the landing receptacle. The friction angle, ρ, is related to the friction coefficient, μ, as:

μ=tan(ρ)

The maximum back pressure, P_(b), see FIG. 3, which can be held by the pressure plug 20 after it has been engaged in the landing receptacle 30 by applied pressure P_(t) can be estimated as:

$P_{b} = {\frac{\tan \left( {\rho - a} \right)}{\tan \left( {\rho + a} \right)} \cdot P_{t}}$

For example, if friction coefficient ρ=0.1 and pressure plug tapering angle α=1.5 degrees, the maximum back pressure P_(b) will be approximately 58% of the pressure P_(t) applied at the engagement of the pressure plug in the landing receptacle. It was found through experimentation and finite element modeling that pressure plugs with tapering angle α from approximately 0.5 to approximately 6 and preferably from 1 to 3 degrees are most useful.

The ratio of the length, L, to the diameter, D, of the tapered section of the pressure plug, see FIG. 2, should be selected based on the maximum expected pressure, P_(t), the yield stresses of the pressure plug and the landing receptacle material, and on the geometry of the landing receptacle. It was found through experimentation and finite element modeling that pressure plugs with L/D ratios of approximately 1 to approximately 6 and preferably of 2 to 4 are most useful.

In this embodiment, pressure plug described above provides both resistance to back pressure and resistance to rotation of the plug relative to the landing receptacle, and creates a metal-to-metal seal between the plug and landing receptacle. However, in cases having the possibility of the tapered area of the plug to be scratched or damaged during pump-down deployment, it is desirable to provide another embodiment with a secondary sealing mechanism. This can be accommodated by incorporating an optional cylindrical section 32, see FIG. 3. The cylindrical section 15 of the plug includes grooves with elastomeric rings 16 adapted to sealingly engage in a cylindrical section of the landing receptacle.

One of the main advantages of the pressure plug system according to present invention compared to conventional pressure plugs is the lack of a slack movement upon cycling of pressure and thus the pressure plug system according to present invention provides a more reliable pressure seal. The other advantage of the pressure plug system according to present invention is its resistance to rotation of the pressure plug during drill-out operations without utilizing locking clutches. Thus, the latching of the plug is more reliable than the latching of conventional anti-rotational plugs.

It should be understood that a pressure plug according to present invention can be made of any suitable material, preferably easily drillable material such as, but not limited to, metal alloys, aluminum, brass, cupper, bronze, phenolic resins, polymeric materials, or reinforced plastics.

The foregoing description and drawings of the invention are explanatory and illustrative thereof, and various changes in sizes, shapes, materials, and arrangements of parts may be made within the scope of the appended claims without departing from the spirit of the invention. 

1. A pressure plug system comprising: a pressure plug comprising a first tapered section with a first tapering angle; a landing receptacle comprising a second tapered section with a second tapering angle substantially equal to the first tapering angle of the first tapered section of the pressure plug; whereby the first tapered section of the pressure plug is adapted to engage in the second tapered section of the landing receptacle; and whereby the first tapering angle of the pressure plug is less than a friction angle corresponding to a coefficient of friction between the first tapered section of the pressure plug and the second tapered section of the landing receptacle.
 2. The pressure plug system of claim 1, wherein the tapering angle is in the range of substantially from 0.5 to 6 degrees.
 3. The pressure plug system of claim 1, wherein the tapering angle is in the range of substantially from 1 to 3 degrees.
 4. The pressure plug system of claim 1, wherein the ratio of the length to the diameter of the tapered section of the pressure plug is substantially from 1 to
 6. 5. The pressure plug system of claim 1, wherein the ratio of the length to the diameter of the tapered section of the pressure plug is substantially from 2 to
 4. 6. The pressure plug system of claim 1 comprising a cylindrical section of constant diameter, wherein the cylindrical section of the pressure plug having at least one groove with elastomeric ring adapted to sealingly engage in a cylindrical section of the landing receptacle.
 7. The pressure plug system of claim 1 comprising at least one elastic cup extending outwardly and rearwardly.
 8. A method of providing a pressure containment comprising: providing a pressure plug system comprising a pressure plug with a tapered section, a receiving landing receptacle having a tapered section with substantially the same tapering angle as said tapered section of the pressure plug, and said tapering angle being less than the friction angle corresponding to the coefficient of friction between the pressure plug and the landing receptacle; pumping said pressure plug down through the drill pipe and landing pressure plug in receiving landing receptacle; applying pressure not less than maximum expected back pressure.
 9. The method of providing a pressure containment of claim 8, wherein said applied pressure is approximately twice higher that the expected maximum back pressure. 